The present invention relates to a signal detecting circuit and, more particularly, to a tone signal detecting circuit.
Conventional tone signal detecting circuits are used for many applications. These circuits can be classified, in terms of basic arrangements, into circuits using analog resonance circuits and circuits using digital signal processors. On the other hand, with the recent advances in LSI technology, a system-on-the-chip has been almost realized, leading to a rapid reduction in size of circuits and apparatuses. Under these circumstances, attempts have been made to reduce the size of a tone signal detecting circuit by using the LSI technology.
In the above-mentioned conventional tone detecting circuits, especially the circuit using an analog resonance circuit, an external part for an LSI is required as a resonance circuit. Even if a resonance circuit is realized within an LSI, since the resultant structure is an analog LSI, the benefit of the advanced micropatterning technique of a digital LSI cannot be enjoyed. Therefore, it is difficult to achieve a further reduction in size. Although the circuit using a digital signal processor can enjoy the benefit of the micropatterning technique of a digital LSI, it requires a processor constituted by gates having at least several kbytes. Therefore, it is difficult to further reduce the size of a circuit on the LSI. In addition, in such a tone detecting circuit, it is important to inhibit detection of tone signals having small amplitudes. That is, it is important to realize a tone detecting circuit which does not detect tone signals having small amplitudes with a simple circuit arrangement.